This invention relates to electrostatographic reproducing apparatus and methods and more particularly to a method and apparatus for transferring toner images from one surface to another.
In the electrostatographic reproducing apparatus commonly in use today, a photoconductive insulating member is typically charged to uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive areas. This image may be subsequently transferred to a support surface such as copy paper to which it may be permanently affixed by heating or the application of pressure. Following transfer of the toner image to the support surface, the photoconductive insulating surface is cleaned of residual toner to prepare it for the next imaging cycle.
In such apparatus it is common to transfer the toner image from the imaging surface to a support surface such as copy paper. Historically, the transfer of toner images to supporting surface such as copy paper is accomplished with electrostatic transfer by either a corotron transfer device or roller electrode biased to transfer potential levels. In corona induced transfer the final support sheet is placed in direct contact with the toner image while the image is supported on the photoconductive surface and the back of the sheet, that is the side of the sheet away from the image is sprayed with a corona discharge having a polarity opposite to that of the toner particles causing the toner to be electrostatically transferred from the photoconductor to the sheet. This system is to a large extent humidity sensitive in that the copy paper which does contain some moisture is sprayed with ions. The moisture in the copy paper makes the paper conductive and charge migrates through the paper thereby distorting the toner image on the paper. This difficulty is compounded by high relative humidity atmospheres since the moisture level of the copy paper is increased.
In bias roll transfer, it is argued that better control of the forces acting on the toner during transfer is had. This type of transfer involves the use of a metal roll with a resistive resilient coating such as that illustrated in U.S. Pat. No. 2,807,233. Because of the resistivity of the coating, the amount of bias that can be applied to the roll is limited to relatively low operating values, since at the higher range the air in and about the transfer zone begins to breakdown, i.e., ionizes causing the image to be degraded during transfer. In addition, in the pre-transfer or pre-nip region before the copy paper contacts the image if the transfer fields are high the image is susceptible to premature transfer across the air gap leading to decreased resolution or fuzzy images. Further, if there is ionization such as mentioned above with regard to the bias roll transfer in the pre-nip air gap from high fields, it may lead to strobing or other image defects, loss of transfer efficiency and a lower latitude of system operating parameters. Yet in the directly adjacent nip region itself, the transfer field should be as large as possible to achieve high transfer efficiency and stable transfer. In the next adjacent post-nip region at the photoconductor copy sheet separation or stripping area, if the transfer fields are too low, hollow characters may be generated. On the other hand, improper ionization in the post-nip region may cause image instability or copy sheet detacking problems. Variations in conditions of copy paper contaminants, etc., can all effect the necessary transfer of parameters. Furthermore, the bias roll material resistivity and paper resistivity can change greatly with humidity. In order to minimize these difficulties various materials have been used in bias roll transfer systems which because of the degree of sophistication of fabricating them are extremely expensive.